package com.github.kezhenxu94.playground.leetcode;

import java.util.Random;

/**
 * <pre>
 *
 * 382. Linked List Random Node
 *
 * Given a singly linked list, return a random node's value from the linked list. Each node must have the same probability of being chosen.
 *
 * Follow up:
 * What if the linked list is extremely large and its length is unknown to you? Could you solve this efficiently without using extra space?
 *
 * Example:
 *
 * // Init a singly linked list [1,2,3].
 * ListNode head = new ListNode(1);
 * head.next = new ListNode(2);
 * head.next.next = new ListNode(3);
 * Solution solution = new Solution(head);
 * 
 * // getRandom() should return either 1, 2, or 3 randomly. Each element should have equal probability of returning.
 * // solution.getRandom();
 * //
 *
 * </pre>
 *
 * @author Ke Zhenxu
 */
public class Solution382LinkedListRandomNode {

	class Solution {
		Random random = new Random();
		int length = 1;
		ListNode p;

		/** @param head The linked list's head.
		 *         Note that the head is guaranteed to be not null, so it contains at least one node. */
		public Solution(ListNode head) {
			p = head;
			while (p.next != null) {
				p = p.next;
				length++;
			}
			p.next = head;
		}

		/** Returns a random node's value. */
		public int getRandom() {
			int rand = random.nextInt(length);
			while (rand-- >= 0)
				p = p.next;
			return p.val;
		}
	}
}
